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Download Article (PDF) Seven Shock-transformed Hexahedrites, Sensitive to Grainboundary Corrosion V. F. Buchwald Department of Metallurgy, Technical University, Lyngby, Denmark Z. Naturforsch. 35a, 361-369 (1980); received January 21, 1980* Dedicated to Professor II. Hintenberger on the occasion of his 70th birthday. Seven polycrystalline iron meteorites (Forsyth County, Holland's Store, Wathena, Pima Co., Chico Mountains, Kopjes Vlei and Mejillones), of bulk composition as normal hexahedrites of group 11 A, are shown to be severely altered hexahedrites. Mic-roprobe examination and various heat treatments of synthetic alloys support the conclusion that the meteorites after a primary slow cooling were subjected to shock-reheating above 800°C. Thereby the following transforma- tions took place a-^-y—>a2—>a, and an unequilibrated structure, very sensitive to grain-boundary corrosion was formed. Introduction Table 1. Weight and bulk composition of the seven polv- crystalline iron meteorites. Other important elements are The last ten years have seen a very significant iron (about 93%), cobalt (0.5%) and sulfur (0.1%). progress with respect to understanding the structure Weight percent PPm and composition of iron meteorites. The efforts in analysing for main and trace elements have resulted kg Ni P Ga Ge Ir in a classification system [1, 2] that may form the Forsyth basis for all further discussions. Mass-spectrometric County 23 5.54 0.21 60 176 31 work [3, 4] has provided valuable insight in the Holland's Store 12.5 5.35 0.25 61 184 20 isotopic composition and the content of noble gases, Wathena 0.57 5.54 0.27 60 184 7.0 data which allow us among other things to deduce Pima the age of meteorites, and the intensity of the cosmic County 0.21 5.60 0.25 60 181 8.9 Chico' radiation. Mountains 0.21 5.56 0.33 59 176 6.2 However, certain meteorites have for various Kopjes Vlei abt. 8 5.65 0.3 59 181 3.1 reasons been difficult to handle. One such group Mejillones abt. 15 5.67 0.33 60 177 2.3 consists of seven polycrystalline iron meteorites, The analyses are weighted averages as reported in [2], Table 1, which because of their enigmatic structure, that sets them apart from the more familiar hexa- hedrite or Widmanstätten structures, only rarely long puzzled scientists, are mainly artefacts, and that have been included in systematic investigations. the extreme sensibility to terrestrial corrosion is due Apart from brief descriptions with photomicrographs to a high degree of unequilibrium, which has sensi- [5, 6] and more extended ones by Buchwald [2], tized the grain boundaries. Finally, an attempt will the meteorites in question have only been cursorily be made to present an evolutionary path that might examined [7 — 9]. In the Hey-Catalogue [10], they lead to the observed structures. are indiscriminately labelled nickel-poor ataxites, brecciated hexahedrites or granular metabolites. Structure The purpose of this paper is to explain the struc- ture and by experiments to support the presentation. While the seven meteorites do have some structural It will be shown that the microscopic holes, that have characteristics that may serve to distinguish them from each other, the following presentation will emphasize what they have in common: The poly- Reprint requests to Dr. V. F. Buchwald, Department of crystalline kamacite, the shock-melted troilite ag- Metallurgy, Technical University, Building 204. 2800 Lyng- by, Dänemark. gregates, the resorbed phosphides, and the general * The first version received October 4, 1979. unequilibration of all structural elements. 0340-4811 I 80 / 377-0370 S 01.00/0. — Please order a reprint rather than making your own copy. 362 V. F. Buchwald • Shock-transformed Hexahedrites Polished sections show a massive material in schreibersite, which once rimmed the troilite nodule, which are embedded scattered diffuse patches of Figure 4. troilite and minute particles of graphite. Graphite occurs as angular or spheroidal, often Upon etching a grain structure, which is unique zoned aggregates, 10 — 150 um across. The spherules to these iron meteorites, develops. Almost equiaxial are composed of anisotropic, radiating crystallites grains, 0.3 — 3 mm in diameter, constitute the bulk and are most common near the schreibersite rem- of the specimens. Each kamacite grain is differently nants, presumably because the graphite was derived oriented as deduced from the oriented sheen from from cohenite, previously rimming the schreibersite. etching, and from the presence of occasional Neu- Figure 5. mann bands. Two of the meteorites, Mejillones and Kopjes Each grain is divided into many thousands almost Vlei, are no more corroded, than the heat-affected a2 equiaxial subgrains, 30 — 50 um in diameter, Fig- zone and an occasional patch of the fusion crust have ures 1 — 2. Within each main-grain the subgrains are been preserved. The others are rather heavily cor- uniformly oriented, since Neumann bands pass the roded. Cohen [11, 12] and others after him, believ- subgrain boundaries with shifts of directions that ed that the strong corrosive attack was due to lawren- are less than 1 . cite, FeClo , a mineral which was assumed to be part The hardness of the metalphase is 190 + 20 of the meteorite structure. As discussed previously (\ickers DPN, 100 g load), somewhat dependent on [13] the chlorine is, however, not of cosmic origin, the local cold work from late splitting in the atmo- but has entered the grain boundaries during a long sphere in connection Avith the fall. exposure to terrestrial groundwater. Grainboundaries and subboundaries are all very Pima County and Chico Mountains, which are dirty, rich in tiny precipitates, 0.2 — 2 um across, verv small and of a fragmental nature, have an ob- which on the microprobe could be shown to be scure origin and may very well have come from the mainly iron-nickel-phosphides. It is, however, pos- same, now lost meteorite [2, p. 976]. sible that some of the smaller ones are carbides. Forsyth County and Holland's Store have, in ad- Equally distributed throughout the sections with dition to the structures discussed above, at one end a frequency of 300 - 500 pr. mm2 there occurs a a different, more hexahedrite-like structure. The two puzzling unit, a rounded or elongated metal grain structural types are separated by a wavy boundary. 3 — 15 um across, normally associated with liny Forsyth County and Holland's Store are in all phosphide particles. These "unequilibrated loops" details so similar that they, too, might come from may be situated on grainboundaries or away from ihe same original fall. them, Figure 3. There are no rhabdite-prisms or -plates in these The problematic pits meteorites. There is, however, occasional large schreibersite crystals, up to 10 X 1 mm in size. These The structural description above was based upon are severely damaged and partially dissolved. the study of perfect sections. However, upon routine Around them are zones with steep gradients of polishing and etching the sections always become nickel and phosphorus, and ihe structure shows a pitted, wherefore most published photomicrographs corresponding zonation in grain size, and frequency (e. g. [5, 6]) show a profusion of black dots. There and type of the tiny precipitates, Figure 8. Nearest has been considerable confusion as to the cause of the phosphide are equal numbers of amoeboid taenite the pits; it has thus been proposed that they rep- particles and angular phosphides, farther away only resented oxides, magnetite [5], altered rhabdites phosphides occur. [2], or were pores in non-massive meteorites [7]. Cm-sized troilite-daubreelite aggregates of the The first result of this investigation is that the usual lamellar development, as seen in e. g. pits are artifacts. They are not genuine and the Coahuila, were once present. Now they have been meteorites are not porous. Careful cutting and transformed into intricate intergrowths of 1 — 10 um polishing, supported by impregnation, and perform- grains of ironsulfide, iron-nickel-sulfide and iron, ed with small load and good cooling and lubrication, with dispersed fragments of daubreelite, and of will usually result in perfect samples with no pitting. Fig. 1. Forsyth County. Corrosion penetrates from the surface (left) along Fig. 2. Forsyth County. The uncorroded interior displays a polycrystalline the grain boundaries. The unequilibrated loops are selectively attacked kamacite structure with a significant number of sensitive, unequilibrated and the polished surface becomes pitted (black). Etched. Scale bar 40 /mi. loops. Lightly etched. Scale bar 40 //m. Fig. 4. Wathena. Detail of a shock-melted troilite nodule. The rapid melting Fig. 3. Forsyth County. Four unequilibrated loops showing various stages and solidification produced a fine-grained eutectic of iron and ironsulfide of destruction, caused by polishing and etching. To the right several in- with dispersed daubreolito (dark grcv) and schreibersite fragments (white), distinct Neumann bands. Oil immersion. Scale bar 10/mu. hitched. Oil immersion. Scale bar 10//in. 364 V. F. Buchwald • Shock-transformed Hexahedrites However, since the pits develop so easily in these are soft, about 145 — 150 Vickers, except when they irons and not in others of the same bulk composition, have been cold-worked by fragmentation in the some explanation must be sought. atmosphere. The perfect polished section is massive and it is The table shows that phosphorus is increased by hardly possible to predict that pits will occur during about a factor five in the pitted meteorites. Simul- subsequent handling. Already a very weak etching taneously the volume-percent of phosphides is signifi- with 1% Nital {1% nitric acid in ethanol) will cantly decreased. Clearly, the kamacite of the poly- reveal the grain- and subgrain boundaries, and their crystalline meteorites is supersaturated with respect associated precipitates. A somewhat longer etching to phosphorus. deepens the grain boundaries, and especially the There are indications that the metallic phases are "unequilibrated loops'' are severely attacked.
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